Relatively high speed interfaces, such as double data rate (e.g., DDR, DDR2, and DDR3, etc.) interfaces may include receivers that use a voltage reference (VREF). An incoming digital signal is compared with the VREF to determine whether or not the input signal is a logic level zero or a logic level one. The voltage level of the VREF acts as a trip point (or switch point). An input signal with a voltage level above the trip point is a logic level one and an input signal with a voltage level below the trip point is a logic level zero.
The regulators that provide the VREF also typically provide the memory device (e.g., a dynamic random access memory device or DRAM) voltage. The VREF is usually fixed at one-half of the DRAM voltage. This arrangement is premised on the DRAM and host having processes that use similar operating voltages. For example, if both the DRAM and the host are based on processes that use an operating voltage of 1.5 V, than the driver signal is likely to be centered (more or less) on a VREF of 750 mV.
In some cases, however, the host that provides the input signal may be based on a process that uses a lower operating voltage than the DRAM. For example, a DRAM may use a standard DRAM voltage (e.g., 1.5 V) but the host may be based on a low-power process that uses a lower operating voltage (e.g., 1.1 V). In such cases, the signal level on the host-side of the interface may not be supported by the receivers on the DRAM side of the interface.